Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent for adult T-cell leukemia. The geographic distribution of HTLV-1 carriers is quite uneven in Japan and the greatest prevalence is in southwestern Japan. Because many people move from endemic areas to the greater Tokyo area, the geographic distribution might have changed. Therefore, we investigated the factors predisposing to HTLV-1 infection, including birthplace, for 88 HTLV-1-infected individuals in greater Tokyo who visited our outpatient clinic. Of these, 39.5% were born in endemic areas, which include Kyushu/Okinawa, south Shikoku, Kii, Tohoku, and Hokkaido, whereas 38.3% were born in greater Tokyo and the proportion is presumed to be increasing. Half of the HTLV-1 infected individuals in greater Tokyo came from endemic areas, whereas around half of the remaining half was presumed to be involved in sexual transmission from a spouse from an endemic area. Overall, they constituted approximately 70% of the HTLV-1 carriers in greater Tokyo. These migration effects may increase the prevalence of HTLV-1 in the greater Tokyo area; nationwide surveillance is warranted.